Plural dos substantivos em inglês

These four nouns don't have a singular and plural form:

Chewing gum,
lipstick,
mascara and
scissors (which also has a silent c, scissors)

NB: To make them countable, we talk about:
a piece of chewing gum,
a stick of lipstick,
a tube of mascara,
a pair of scissors.

Fonte: ETP - conselho britânico